Irradiation-induced osteocyte damage promotes HMGB1-mediated osteoclastogenesis in vitro.

Irradiation-induced bone loss is widely reported, especially in radiotherapy-induced osteoporosis. In addition to the mechanism of osteogenesis inhibition and osteoclastogenesis promotion, the regulation effect of osteocytes, which also send signals to modulate osteoclastogenesis, should be elucidated. In this study, the effect of irradiation on osteocyte and its accommodation to osteoclastogenesis via the release of high mobility group box 1 (HMGB1) was explored. Furthermore, the control response of HMGB1 inhibitor on receptor activator of nuclear factor-κB ligand (RANKL) and osteoprotegerin (OPG) expression in osteocyte and osteocyte-induced osteoclastogenesis was assessed. It was observed that irradiated osteocyte-like MLO-Y4 cells exhibited polygonal-shaped morphological changes and shortened dendrites, inhibited cell viability and induced cellular apoptosis, along with the reduction in dendritic E11 protein/messenger RNA expression at a doses of 4 Gy. Additionally, the secretion of HMGB1 in supernatants was promoted, accompanied by the decreased OPG and elevated RANKL expression. When the RAW264.7 cells were cocultured with irradiated MLO-Y4 cells or its conditioned medium, enhanced migration and differentiation of osteoclast precursor was observed, and this difference was alleviated with anti-HMGB1 neutralizing antibody. In conclusion, this study demonstrated that irradiation deteriorated osteocytes' potential to promote recruitment and differentiation of osteoclast precursor via stimulating HMGB1 release and subsequent elevation of RANKL/OPG level. This study will assist in designing the intervention programs for irradiation-induced bone loss.